First-overtone Cepheid mass determination

First determination of the mass and the projection factor for the first-overtone Cepheid OGLE-LMC-CEP-2532 in a binary system.

The Araucaria Project: The First-Overtone Classical Cepheid in the Eclipsing System OGLE-LMC-CEP-2532

B. Pilecki, D. Graczyk, W. Gieren, G. Pietrzyński, I. B. Thompson, R. Smolec, A. Udalski, I. Soszyński, P. Konorski, M. Taormina, A. Gallenne, D. Minniti, M. Catelan

2015, ApJ, 806, 29

We present here the first spectroscopic and photometric analysis of the double-lined eclipsing binary containing the classical, first-overtone Cepheid OGLE-LMC-CEP-2532 (MACHO 81.8997.87). The system has an orbital period of 800 days and the Cepheid is pulsating with a period of 2.035 days.

Using spectroscopic data from three high-class telescopes and photometry from three surveys spanning 7500 days we are able to derive the dynamical masses for both stars with an accuracy better than 3%. This makes the Cepheid in this system one of a few classical Cepheids with an accurate dynamical mass determination (M1=3.90 ± 0.10 M). The companion is probably slightly less massive (3.82 ± 0.10 M), but may have the same mass within errors (M2/M1= 0.981 ± 0.015). The system has an age of about 185 million years and the Cepheid is in a more advanced evolutionary stage.

For the first time precise parameters are derived for both stars in this system. Due to the lack of the secondary eclipse for many years not much was known about the Cepheid’s companion. In our analysis we used extra information from the pulsations and the orbital solution from the radial velocity curve. The best model predicts a grazing secondary eclipse shallower than 1 mmag, hence undetectable in the data, about 370 days after the primary eclipse.

The dynamical mass obtained here is the most accurate known for a first-overtone Cepheid and will contribute to the solution of the Cepheid mass discrepancy problem.

Light curve

I-band OGLE-IV data (points) and the model (solid line). No systematics in residuals is visible in the eclipse. The apparent beating of the observations is caused by the pulsating period being almost equal to 2 days. 

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